Question 1129621
z=(x-mean)/sd
=(50-42)/24 or here z>(1/3) That is 0.3696 or 370 students (using z=+0.33)
Between 30 and 54 is between z of -0.5 and +0.5, which is a probability of 0.3829 or 383 students (that is 12 points on either side of the mean and 12/24 is 1/2, dividing by the sd)
Top 100 students is at the 90th percentile z=+1.28 
using the formula above z*sd=30.72
That is x-mean, so x=mean+30.72 or a score 72.72 or 73 to the nearest integer.